Twisted microwave applicator

ABSTRACT

A twisted microwave applicator is provided to avoid the heating non-uniformities inherently resulting from non-twisted microwave applicators. The invention comprises a section of waveguide twisted about its axis, preferably twisted 90* or more. The applicator is coupled to a microwave generator by conventional techniques. The objects to be treated are introduced into one end of the applicator, transported through the applicator and removed at the other end. The twisted waveguide provides for rotation of the field pattern, which tends to subject all portions of the objects to the same overall heating effect.

United States Patent [191 Jory et al.

TWISTED MICROWAVE APPLICATOR [75] Inventors: Howard R. Jory, Menlo Park;

Charles H. Will, Jr., San lose, both of Calif.

[73] Assignee: Varian Associates, Palo Alto, Calif. [22] Filed: Sept.21, 1973 [21] Appl. No.: 399,676

Related U.S. Application Data [63] Continuation of Ser. No. 208.768,Dec. 16. i971,

abandoned.

[52] U.S. Cl. 219/1055, 333/98 R [51] Int. Cl. H05b 9/06 [58] Field ofSearch 219/1055; 333/98 R [56] References Cited 'UNITED STATES PATENTS 37l5,55l 2/1973 Petersod 219/1055 [4 Oct. 22, 1974 Primary Examiner-J. V.Truhe Assistant Examiner-Hugh D. Jaeger Attorney, Agent, or FirmStanleyZ. Cole; Leon F. Herbert; John J. Morrissey 5 7 ABSTRACT A twistedmicrowave applicator is provided to avoid the heating non-uniformitiesinherently resulting from non-twisted microwave applicators. Theinvention comprises a section of waveguide twisted about its axis,preferably twisted 90 or more. The applicator is coupled to a microwavegenerator by conventional techniques. The objects to be treated areintroduced into one end of the applicator, transported through theapplicator and removed at the other end. The twisted waveguide providesfor rotation of the field pattern, which tends to subject all portionsof the objects to the same overall heating effect.

14 Claims, 5 Drawing Figures PATENTEDncr 22 m4 FIG.2

smniorz I This is a continuation of application Ser. No. 208,768 filedDec. 16, 1971 now abandoned.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention isa further deveopment in the field of microwave treatment applicators,particularly in the technique of providing a uniform heating effect onall parts of an object passing through the applicator.

2. Description of the Prior Art In conventional microwave applicators,the electric field varies in intensity over any given cross section ofthe applicator at any given instant of time depending upon the distancefrom the conductive walls of the applicator to the point at which theelectric intensity is measured. In addition, further variations in fieldintensity will be introduced over the given cross-section because offield distortions caused by the dielectric properties of the objects tobe treated. An object passing through a conventional microwaveapplicator will experience different heating effects on its varioussurfaces and internal portions because of these nonuniformitiesin theelectric field intensity which are inherent in conventional applicators.

SUMMARY OF THE INVENTION It is an object of this invention to providefor the exposure of all surfaces and internal portions of materialtreated in a microwave applicator to a heating effect of increaseduniformity within the applicator.

It is a further object of this invention to achieve a heating effect ofincreased uniformity within a microwave applicator by providing forrelative rotation between the field pattern in the applicator and thematerial to be treated.

It is a further object of this invention to provide for rotation of thefield pattern within a microwave applicator by means of a twistedwaveguide section.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a perspective view of atwisted microwave applicator illustrating the invention embodied in aresonant cavity applicator;

FIG. 2 is a cross-section of the applicator taken on line 2--2 of FIG.1;

FIG. 3 illustrates the non-uniformity of the electric field intensityover a cross-section of a microwave applicator;

FIG. 4 illustrates the distortion of the electric field intensity in amicrowave cavity caused by the dielectric properties of an object beingtreated; and

I FIG. 5 is a view similar to FIG. 1 illustrating the invention embodiedin a traveling wave applicator.

DESCRIPTION OF THE INVENTION 2 is rectangular in cross section at everypoint along its length as shown for example in FIG. 2. Although arectangular cross-section is preferred, the principle of the inventionapplies to any cross-section shape which tends to increase uniformity ofheating when it is twisted. The objects to be treated within the cavityare introduced and removed through openings in the end walls. In oneapproach the objects are introduced through a tube 5 and removed througha tube 6. Tubes 5 and 6 are made of conductive material and are designedin conventional manner to be non-propagating at the frequency ofoperation of the applicator so that microwave energy will not escapefrom the cavity. Tubes 5 and 6 are secured to the end walls but do notextend inside the waveguide 2. The objects to be treated are preferablyenclosed inside the cavity by a dielectric tube 7 extending between theend walls and coaxial with tubes 5 and 6. In this way the objects to betreated can be inserted into tube 5 and push one another through thetube 7 and out tube 6. Also, the waveguide section 2 can be oriented toslope downward from the input to the output end to provide a gravitytransport. Another alternative is to run a conventional conveyer belt(not shown) through the waveguide section 2 to carry the objects. Iftube 7 is employed, the belt would go through the tube. If the ac tionof the belt transport and the shape of the objects would tend to causerandom rotation of the objects on top of the belt, the objects should beclipped or otherwise fastened to the belt to avoid the possibility thatsome objects might rotate in a way which would tend to offset thetwisting effect of the twisted waveguide. Waveguide section 2 is coupledto a microwave generator (not shown) by a conventional inlet waveguidesection 10 coupled to waveguide 2 through an iris opening 11.Alternatively, coaxial line and coupling loop type of coupling can beemployed although such a coupling would be made in a wide wall ofwaveguide section 2, rather than in a narrow wall.

Waveguide section 2 is twisted to achieve rotation of theelectromagnetic field pattern from one end of the microwave treatmentcavity to the other. This rotation of the field pattern will cause thesurfaces and internal portions of objects passing through the treatmentcavity to experience a unifonn heating effect. The heating effect at anypoint within the cavity is proportional to the square of the electricfield strength at that point. If there are local variations in the fieldstrength within the cavity, then objects having different shapes,objects having different paths through the waveguide, and differentportions of a specific object will experience nonuniform heating effectsunless all portions of each object are exposed to the same net fieldintegrated over the path of the object through the cavity. In aconventional straight waveguide section the non-uniformity remainsconstant along the section, so for example, a portion of an object whichis exposed to a low energy field, is exposed to the same low fieldthroughout its travel through the waveguide. -By rotating the fieldpattern about an object as it passes through the cavity, this inventionprovides the desired integration so that all objects and all parts ofthe same object tend to experience the same net exposure to themicrowave energy within the cavity as they pass through the cavity.

FIG. 3 illustrates the variation in electric field intensity over thecross section of a rectangular microwave applicator (such as anycross-section of waveguide section 2) at any given instant of time. Thefield variation 'is shown by individual field vector lines E and theline 12 which depicts the envelope of the changing field strength acrossthe waveguide section. This variation results from the fact that thecomponents of the electric field parallel to the narrow conducting wallsdecrease in magnitude as the distance from the narrow walls decrease,until E at the narrow walls. FIG. 4 illustrates the distortion of theelectric field intensity over a given cross-section caused by thenon-uniform cross section or non-uniform dielectric properties of anobject 14 to be treated. The non-uniformities in the electric fieldintensity such as illustrated in FIG. 3 and FIG. 4 cause non-uniformheating effects upon objects passing through a microwave applicator inwhich the field pattern remains constant. However, in accordance withthis invention, rotating the field pattern about the objects as theypass through the applicator, causes the objects to experience changingfields which tend to provide a uniform heating effect on the objects inthe course of their travel through the applicator. As indicated by the Earrows in FIG. 1 the applicator formed by the 90 twisted waveguidesection 2 provides an E field which changes from vertical at the inletend of section 2 to horizontal at the outlet end.

Use of a straight waveguide section in which the material to be treatedis rotated in controlled manner as it passes through the waveguidesection can be used to simulate the result achieved with a twistedwaveguide. However, this requires a complex mechanical system forrotating the material. The complexity of such a system involvesinherently poorer reliability and greater maintenance problems than thetwisted waveguide.

Although the 90 twist of waveguide section 2' 'is shown in FIG. l, itwill be understood that twisting of more or less than 90 can be employedto achieve heating uniformity which is better than with a straightwaveguide. However, at least 90 twist is required to cause I theelectric field lines to pass through the objects parallel to thevertical axis of the objects and also parallel to the horizontal axis ofthe objects. Where more than 90 is used, the twist is preferably inmultiples of 90, and preferably the number of multiples is selected sothat the object to be treated is not exposed to electrical field more inone direction than in another direction. As is known by those skilled inthe art, the electrical field axially along the resonant cavityapplicator l is in form of a sine wave with zero intensity at the endwalls. Thus if the twist starts immediately at the end walls 3 and 4,the full effect of the twist is not realized. Accordingly it ispreferred to have straight waveguide lengths L at the inlet and outletends, particularly where a twist of only 90 is used. Although any lengthL is beneficial, the straight end portions preferably have a length L onthe order of A of the wavelength of the waveguide wavelength, asdistinguished from free space wavelength.

It is also possible to use a twisted waveguide section in a travelingwave applicator as shown at 1' in FIG. 5. The differences between theresonant cavity applicator ,1 of FIG.;1 and the traveling waveapplicator 1' in FIG.

2 are that the traveling wave'applicator has a waveguide output 16 whichgoes to a dummy load (not shown), and the coupling fromthe inletwaveguide opens completely into the waveguide section 2 so thatmicrowave energy passes into section 2 through the fully open end ofsection 10 rather than through the small iris hole 11. In the travelingwave embodiment, the field strengths lengthwise along section 2 are notheld to zero at the end walls 3 and 4 so the straight end sections L arenot required for the reason discussed in connection with FIG. 1.However, as a practical matter straight sections L are preferred in thetraveling wave case to simplify connection of the input and outputsections 10.

What is claimed is:

1. An electromagnetic energy applicator comprising a waveguide sectionelongate about an axis and having electromagnetic energy input meansconnected thereto, said waveguide section having two rectangular endwalls with openings therein forpassage of an object along a path throughsaid waveguide section, said end walls being substantially opague tosaid electromagnetic energy, said waveguide section being configured sothat the longer rectangular dimension of one of said end walls isnonparallel to the longer rectangular dimension of the other of said endwalls, said waveguide section being further configured so that as saidobject passes along said path from said one end wall to said other endwall said object is exposed for the greater portion of said path to acontinuously varying orientation of the electric field of saidelectromagnetic energy.

2. The applicator of claim 1 wherein said waveguide section forms aresonant cavity; and wherein said waveguide section comprises a twistedportion interposed between two nontwisted portions; said end walls beingdisposed to permit passage of said object sequentially through one ofsaid non-twisted portions, then through said twisted portion, and thenthrough the other of said non-twisted portions along said path; each ofsaid nontwisted portions having a length along said path sufficient toprovide a non-zero electric field intensity at the boundary between saidnon-twisted portion and said twisted portion; said twisted portionhaving a length along said path sufficient to provide that said objectis exposed to more of said energy in said twisted portion than in saidnon-twisted portions. v

3. The applicator of claim 2 wherein said twisted portion of saidwaveguide section is twisted by at least 4. The applicator of claim 2wherein the length of eachof said non-twisted portions of said waveguidesection is on the order of one quarter of the waveguide wavelength ofsaid electromagnetic energy.

5. The applicator of claim 1 wherein said waveguide section comprises atwisted portion interposed'between two non-twisted portions; said endwalls being disposed to permit passage of said object sequentiallythrough one 'of said non-twisted portions, then through said twistedportion, and then through the other of said nontwisted portions alongsaid path; the length of each of said non-twisted portions along saidpath being sufficient to provide maximum electric field intensity ofsaid energy at the boundary between said non-twisted portion and saidtwisted portion. i

6. The applicator of claim 5 wherein the'length of each of saidnon-twisted portion along said path is substantially no longer thanone-quarter of the waveguide wavelength of said energy;

7. The applicator of claim 1 wherein said applicator further compriseselectromagnetic energy output means connected to said waveguide section;said waveguide section comprisingatwisted portion interposed between twonon-twisted portions; said end walls being disposed to permit passage ofsaid object sequentially through one of said nontwisted portions, thenthrough said twisted portion, and then through the other of saidnon-twisted portions along said path; one of said nontwisted portionshaving a length sufficient to accommodate thereon the connection of saidelectromagnetic energy input means to said waveguide section and theother of said non twisted portions having a length sufficient toaccommodate thereon the connection of said electromagnetic energy outputmeans to said waveguide section; said twisted portion having a lengthalong said path sufficient to provide that said object is exposed tomore of said energy in said twisted portion than in said nontwistedportions.

8. The applicator of claim 7 wherein said twisted portion of saidwaveguide section is twisted by at least 90.

9. The applicator of claim 1 wherein said waveguide section comprises atwisted portion interposed between two non-twisted portions; said endwalls being disposed to permit passage of said object sequentiallythrough one of said non-twisted portions, then through said twistedportion, and then through the other of said nontwisted portions alongsaid path; the length of each of said non-twisted portions along saidpath being sufficiently small to provide that said object is exposed tomore of said energy in said twisted portion than in said non-twistedportions.

10. The applicator of claim 1 wherein said waveguide section forms aresonant cavity.

11. The applicator of claim 1 wherein said rectangular end walls areperpendicular to said axis of said waveguide section.

pendicular to said axis of said energy input waveguide. =l= I=

1. An electromagnetic energy applicator comPrising a waveguide sectionelongate about an axis and having electromagnetic energy input meansconnected thereto, said waveguide section having two rectangular endwalls with openings therein for passage of an object along a paththrough said waveguide section, said end walls being substantiallyopague to said electromagnetic energy, said waveguide section beingconfigured so that the longer rectangular dimension of one of said endwalls is nonparallel to the longer rectangular dimension of the other ofsaid end walls, said waveguide section being further configured so thatas said object passes along said path from said one end wall to saidother end wall said object is exposed for the greater portion of saidpath to a continuously varying orientation of the electric field of saidelectromagnetic energy.
 2. The applicator of claim 1 wherein saidwaveguide section forms a resonant cavity; and wherein said waveguidesection comprises a twisted portion interposed between two nontwistedportions; said end walls being disposed to permit passage of said objectsequentially through one of said non-twisted portions, then through saidtwisted portion, and then through the other of said non-twisted portionsalong said path; each of said non-twisted portions having a length alongsaid path sufficient to provide a non-zero electric field intensity atthe boundary between said non-twisted portion and said twisted portion;said twisted portion having a length along said path sufficient toprovide that said object is exposed to more of said energy in saidtwisted portion than in said non-twisted portions.
 3. The applicator ofclaim 2 wherein said twisted portion of said waveguide section istwisted by at least 90*.
 4. The applicator of claim 2 wherein the lengthof each of said non-twisted portions of said waveguide section is on theorder of one-quarter of the waveguide wavelength of said electromagneticenergy.
 5. The applicator of claim 1 wherein said waveguide sectioncomprises a twisted portion interposed between two non-twisted portions;said end walls being disposed to permit passage of said objectsequentially through one of said non-twisted portions, then through saidtwisted portion, and then through the other of said non-twisted portionsalong said path; the length of each of said non-twisted portions alongsaid path being sufficient to provide maximum electric field intensityof said energy at the boundary between said non-twisted portion and saidtwisted portion.
 6. The applicator of claim 5 wherein the length of eachof said non-twisted portion along said path is substantially no longerthan one-quarter of the waveguide wavelength of said energy.
 7. Theapplicator of claim 1 wherein said applicator further compriseselectromagnetic energy output means connected to said waveguide section;said waveguide section comprising a twisted portion interposed betweentwo non-twisted portions; said end walls being disposed to permitpassage of said object sequentially through one of said nontwistedportions, then through said twisted portion, and then through the otherof said non-twisted portions along said path; one of said non-twistedportions having a length sufficient to accommodate thereon theconnection of said electromagnetic energy input means to said waveguidesection and the other of said non-twisted portions having a lengthsufficient to accommodate thereon the connection of said electromagneticenergy output means to said waveguide section; said twisted portionhaving a length along said path sufficient to provide that said objectis exposed to more of said energy in said twisted portion than in saidnontwisted portions.
 8. The applicator of claim 7 wherein said twistedportion of said waveguide section is twisted by at least 90*.
 9. Theapplicator of claim 1 wherein said waveguide section comprises a twistedportion interposed between two non-twisted portions; said end wallsbeing disposed to permit passage of said object sequeNtially through oneof said non-twisted portions, then through said twisted portion, andthen through the other of said non-twisted portions along said path; thelength of each of said non-twisted portions along said path beingsufficiently small to provide that said object is exposed to more ofsaid energy in said twisted portion than in said non-twisted portions.10. The applicator of claim 1 wherein said waveguide section forms aresonant cavity.
 11. The applicator of claim 1 wherein said rectangularend walls are perpendicular to said axis of said waveguide section. 12.The applicator of claim 1 wherein said electromagnetic energy inputmeans comprises an energy input waveguide which is electromagneticallycoupled to said object-treating waveguide section through an irisopening.
 13. The applicator of claim 1 wherein said electromagneticenergy input means comprises an energy input waveguide which is elongateabout an axis, said energy input waveguide axis being perpendicular tothe axis of said object-treating waveguide section.
 14. The applicatorof claim 13 wherein the interface between said energy input waveguideand said object-treating waveguide section lies in a plane which isperpendicular to said axis of said energy input waveguide.